1. Field of the Invention
This invention pertains to optical channels and more particularly to low loss, high isolation, in-plane crossover structures for the intersection of two optical channels, optical channel bends and a low loss, high isolation optical channel switch.
2. Description of the Prior Art
During the assembly of an optical system, it may be necessary to cross two light guiding channels existing in a common planar single crystal platelet without coupling light signals from one channel to the other channel. Crossovers of this type in the prior art presented fabrication difficulties, requiring that the light guiding channels remain separate and distinct, thus necessitating a transition of at least one of the channels from the initial common plane to another plane in which only one of the crossing channels is situated or conversely did not provide sufficient isolation when the channels cross at small angles with respect to one another.
Refer to FIG. 1 wherein is shown, in schematic form, an in-plane crossover of two light guiding channels as would be fabricated without the present invention. Light guiding channels A and B have equal refractive indices and form an angle .PHI. in the crossover plane. The refractive index N of the remaining planar material C is selected such that light diverging within the angular limits of -.theta. and +.theta., with respect to channels A and B, are constrained within the channels A and B respectively. An optical wave incident to the crossover region from one of the channels (channel A in FIG. 1) may couple a portion of the light energy to the second channel (channel B in FIG. 1) which lies at an angle .gamma. with respect to the direction of the second channel. When .PHI. is less than 2.theta., .gamma. may be less than .theta. and a portion of the light energy propagating in the first channel may enter and propagate down the second channel to an output port thereof. When .PHI. is greater than 2.theta., .gamma. will in all cases be greater than .theta. and the portion of the light energy coupled into the second channel, rather than propagating therein, will be radiated through the channel boundary D into the remaining planar material C. In the latter situation, cross coupling between the channels has been eliminated at the expense of an attenuation (due to the radiation loss at the channel crossover) of the light signal propagating in the first channel.
It is desirable to have an in-plane optical waveguide crossover structure that passively provides channel isolation without an attendant loss of light energy.